1. Field of the Invention
The present invention relates to a sheet stacking apparatus which aligns and stacks sheets and a sheet processing apparatus having the same.
2. Description of Related Art
In conventional sheet processing apparatuses, an apparatus which processes the sheet stacked on an intermediate processing tray after performing the alignment in the conveying direction by a plurality of return paddles and a trailing end stopper and the alignment in the width direction perpendicular to the sheet conveying direction by a lateral aligning member has been proposed (refer to Japanese Patent Application Laid-Open No. 11-199121).
Further, there is an apparatus which can reliably allow the sheet to be drawn to the stopper by changing an operating time of pulling of the return paddle which performs the alignment in the conveying direction between a sheet in a different alignment process and a sheet other than the final sheet, and thus avoids the misalignment of the sheet (refer to JP-A No. 2002-154731).
Furthermore, there is an apparatus which keeps the contact area between the topmost sheet and the return paddle always constant in order to prevent the sheet from being excessively returned due to changes of the contact area of the return paddle (refer to U.S. Pat. No. 6,220,592). Conventionally, the trailing end of the sheet is abutted toward the trailing end stopper by rotating such a return paddle during a sheet discharge interval. However, the sheet discharge interval varies depending on the size of the sheet. Thus, it is necessary to control each sheet so that it is discharged within each of the discharge intervals. For example, a small size sheet (e.g. A4, LTR, etc.) whose discharge interval is short is rotated one rotation while a large size sheet (e.g. A3, LDR, LGL, etc.) whose discharge interval is long is rotated two rotations.
Further, a sheet with a large mass like a large size sheet (namely, a sheet having a large inertial force) is required to be returned by a large conveying force. Therefore, the rotating speed of the return paddle is changed according to the size of the sheet. When the mass of the sheet is large, the conveying force of the sheet is increased by increasing the rotation number of the paddle. The denting, folding, and bounce of the trailing end of the sheet when the sheet is abutted against the trailing end stopper is suppressed by rotating the return paddle at a uniform speed. The rotating speed of the return paddle at this time is set to 700 mm/s to 800 mm/s.
In some cases, the conventional sheet processing apparatuses align a thin paper (grammage; approximately 40 to 52 g/m2) whose grammage is lower than that of a plain paper for printing (grammage; approximately 64 to 80 g/m2). In the case where a thin paper whose sheet stiffness is small is abutted against the trailing end stopper at the rotating speed of the return paddle (700 to 800 mm/s) similar to that of the plain paper and aligned, the paper may be buckled when it is abutted against the trailing end stopper. As a result, alignment properties of the thin paper may be reduced. As with the case of the thin paper, when the plain paper for printing is abutted against the trailing end stopper, the plain paper may be buckled depending on the use environment.
For example, the stiffness of the plain paper is decreased by moisture absorption effect of the sheet when used under a high-humidity condition. The stiffness is the same as that of the thin paper and thus the plain paper may be buckled. When used under a low-humidity condition, the end portion of the sheet has a curled-up shape (hereinafter referred to as curl). When the sheet is curled in the front-back direction in the sheet conveying direction, the elasticity of the end to be abutted against the trailing end stopper becomes poor and thus the sheet is easily buckled.
Even if the small size sheet can be abutted against the trailing end stopper by giving the return paddle one rotation, when the large size sheet is used, the sheet may not be returned to the trailing end stopper unless the paddle is rotated two rotations. This is because the inertial force is large. Therefore, the rotating speed of the return paddle is uniformly increased in order to give the paddle two rotations during the sheet discharge interval. However, since the return paddle is rotated twice during the short discharge interval of the sheet, the denting, folding, and bounce of the trailing end of the sheet can be caused when the return paddle is speeded up to rotate twice.
As described above, when conventional sheet stacking apparatuses are connected to an output apparatus such as an image forming apparatus having a high productivity, a sheet discharge interval which can be processed is limited and further a high-speed aligning operation is difficult. There has been a difficulty in handling a wide variety of sheets such as thin paper. With reference to the use environment, the use under a wide range of environments has been difficult.
The present invention provides a sheet stacking apparatus and a sheet processing apparatus which can improve aligning properties, stacking properties, and discharge productivity regardless of the sheet type, the sheet size, and the use environment.